This invention relates to methods for injection molding and extruding wet hygroscopic ionomers, e.g., Surlyn.RTM. (registered trademark of E.I. DuPont deNemours & Co., Wilmington, Del.), to prepare ionomer components having excellent properties.
It is well known that a wide variety of thermoplastic polymers can be injection molded and/or extruded to prepare polymeric components now ubiquitous in commerce. Moisture contained in such thermoplastic resins can have deleterious effects on the properties of the plastic components if not removed from the polymer prior to the point of injection into a mold or extrusion through a die. Many ways are known or have been suggested to remove absorbed water from thermoplastic resins either prior to or during the injection molding or extrusion process.
Two drying methods are common. Drying ovens or hopper dryers for feeding the thermoplastic resins have been successfully employed for many polymers. These often are problematic or too expensive. An alternative approach is to use a vented barrel for the injection molding or extrusion operation. See, e.g., U.S. Pat. Nos. 3,826,477, 4,155,655, 4,185,060, 4,367,190 and many others.
A Wide variety of screws and operating conditions can be used in conjunction with vented barrels in both injection molding and extrusion methods. Many of the considerations important in designing systems for vented barrel thermoplastic processing are discussed in Dupont Technical Report, Number 201, "Molding of Dupont Engineering Thermoplastics Using Vented Barrel Reciprocating Screw Injection Machines," W.P. Filbert, July, 1979, whose entire disclosure is incorporated by reference herein. One type of screw discussed by Filbert is the barrier screw, many designs for which are known. See, for example, U.S. Pat. Nos. 3,375,549, 3,698,541, 4,128,341, 4,341,474, etc., EPA 0,034,505, "Screw and Barrel Technology", 1985, Spirex Corporation, 8469 Southern Boulevard, P.0. Box 3409, Youngstown, Ohio 44512, and the like, whose entire disclosures are incorporated by reference herein. Many other barrier screw designs are known.
A two-stage "barrier screw" is a special type of two-stage screw. In a normal two-stage screw (see FIG. 1), the first stage comprises the initial feeding section, a first transition zone and a first metering zone. The second stage comprises a vent section, followed by a second transition zone and a second metering zone. In a two-stage barrier screw, a section of the screw, usually the first transition section, is split into two different channels by the initiation of a secondary flight, typically at the beginning of the transition section. See FIGS. 2 and 3. The forwardmost of the two progressing adjacent channels is a solids channel; the rearmost is a melt channel. As the solids melt in the leading channel, the melted polymer passes over the top of the barrier flight into the melt channel whereby the solid and melt phases are maintained essentially separate from one another. This results in significant advantages, including more efficient, uniform and complete melting of the solids, more efficient refinement of the melted polymer with lower shear and overheating, etc.
Although the foregoing drying techniques have been successfully applied commercially to many thermoplastic polymers, there still exist certain classes of polymers which have not been adequately dryable just prior to or during injection molding and/or extrusion. One class of thermoplastic polymers to which these conventional techniques have been inapplicable is that of the ionomeric polymers (ionomers), such as the polyethylene ionomers typified by Surlyn.RTM..
The problem for ionomers such as Surlyn.RTM. has been so severe that it has heretofore been commercially injection moldable and extrudable only when provided dry in air-tight containers. Otherwise, these very hygroscopic ionomers quickly absorb moisture from the atmosphere which precludes successful injection molding and extrusion using any known methods. When attempts have been made to injection mold or extrude Surlyn.RTM. having a water content at or above the water content of Surlyn.RTM. when exposed to ambient conditions, totally unacceptable results occur. In conventional vented screw devices used in the past, the unusual combination of properties of ionomers (low softening temperatures (about 160.degree. F.), high tackiness, high viscosity in the melt phase, high expandability, etc.) makes vent flooding and system plugging unavoidable, adequate drying unobtainable and significantly reduced rates a necessity where barely tolerable results might be achieved. Even simple hopper drying has been inapplicable due in part to the low vapor pressures achieved at the low softening temperatures typically above 150.degree. F. Normally, much higher temperatures are available on other materials for drying.
Moreover, hopper jamming and plugging occur in hopper dryers. Where molded or extruded products are nevertheless obtained, they have been unacceptable because of the defects caused by contained moisture, e.g., splaying, voids, etc. Furthermore, other properties change as a function of the amount of water absorbed, making thermoplastic processing unreliable and unpredictable. These include changes in rheology which preclude acceptable component size control, changes in chemical reactivity with additives, changes in optical properties, etc.
Consequently, heretofore it has been necessary commercially to supply and maintain ionomers such as Surlyn.RTM. in dry form, using hermetically sealed, moistureproof containers. This requirement imposes a significant cost increase over the more conventional techniques of shipping thermoplastic resins in much larger lots under ambient conditions. Furthermore, even with this safeguard, extruders or injection molding machines processing dry ionomers can often be interrupted, e.g., because of cycle problems, mold adjustments, etc. Any unused ionomer exposed to the atmosphere is then usually discarded because it will quickly absorb sufficient moisture to make it thereafter unusable for the reasons discussed above. Similarly, it is usually not possible to regrind Surlyn.RTM. and other ionomeric components.
Because of the foregoing state of the art, ionomers have never successfully been injection molded in wet form. There has been some limited success in the extrusion process using vented barrel extruders. However, this success has only been for a given resin under a limited set of conditions using a specific screw design. Any change in the properties of the resin, processing conditions or screw design has resulted in failure in the past. No system exists wherein thermoplastic processing can be conducted using a wide variety of ionomer resins under a wide variety of conditions.
This inability to dry moist ionomers or to thermoplastically process them commercially in wet form has plagued their injection molding and extruding applications. This has continued to be a severe problem despite improvements in state of the art knowledge concerning venting of a wide variety of thermoplastic resins.